One of factors required to allow an automobile to run safely is a tire air pressure. When the air pressure is lower than an appropriate value, the stable maneuverability or fuel consumption is deteriorated, which may cause a tire burst. Thus, Tire Pressure Monitoring System (TPMS) for detecting a tire having a decreased pressure to send an alarm to the driver to prompt an appropriate action is an important technique from the viewpoint of environment protection and driver safety.
A conventional alarm apparatus can be classified into two types of the direct detection-type one (direct TPMS) and the indirect detection-type one (indirect TPMS). The direct TPMS provides a pressure sensor in a tire wheel to thereby directly measure the tire pressure. The direct TPMS can detect a decrease in the pressure at a high accuracy but has a problematic fault-tolerance performance for example and thus is still disadvantageous in the technical and cost aspects.
On the other hand, the indirect TPMS is a method of estimating the air pressure based on the tire rotation information. The indirect TPMS can be further classified into the Dynamic Loaded Radius (DLR) method and the Resonance Frequency Mechanism (RFM) method. The DLR method is a method that uses a phenomenon according to which a tire having a decreased pressure in a running vehicle is collapsed and thus the tire has a reduced dynamic loaded radius and is consequently rotated at a higher speed than other tires having a normal pressure. The DLR method compares the rotation rates of the four tires to thereby detect a tire having a decreased pressure. Since this method can use only wheel rotation speed signals obtained from a wheel speed sensor to subject the signals to a relatively-easy computation processing, this method has been widely researched mainly for the purpose of detecting a puncture of one wheel. However, this method merely makes a relative comparison among wheel rotation speeds and thus cannot detect a case where the four wheels simultaneous deflation (natural leakage). Furthermore, a disadvantage is caused where a decreased pressure cannot be accurately detected through all running conditions because a difference in the wheel speed is caused also by running conditions such as the turning of the vehicle, the acceleration and deceleration, and an eccentric load.
On the other hand, the RFM method is a method to use a fact that a tire having a decreased pressure has a different wheel speed signal frequency characteristic to thereby detect a difference from a normal pressure. In contrast with the DLR method, the RFM method is an absolute comparison with the normal values of the respective wheels that are retained in advance. Thus, the RFM method also can detect a case of the four wheels simultaneous deflation. Thus, the RFM method attracts attentions as a better indirect detection method. However, the RFM method has a disadvantage where some running conditions cause strong noise for example and thus an estimated frequency value of a target domain is not robust against the vehicle speed and the road surface situation for example. The present invention relates to an apparatus for detecting a tire status based on the RFM method. Hereinafter, the basic principle of this method will be described in more detail.
When the vehicle is running, the tires receive a force from the road surface to thereby cause the torsional motion in the front-and-rear direction and the front-and-rear motion of the suspension, and these motions have a coupled resonance vibration. Since this resonance phenomenon also has an influence on the wheel rotation motion, a wheel speed signal obtained from a wheel sensor provided in the Anti-Lock Braking System (ABS) also includes information related to the resonance phenomenon. Furthermore, since the coupled resonance vibration is caused in a unique vibration mode due to the tire torsional rigidity, the excitation status thereof changes so as to depend only on a change in the air pressure constituting the tire physical characteristic and has a very small dependence on a change in the vehicle speed and a change in the road surface. Specifically, a decreased air pressure causes a change in the dynamics of the tire torsional motion. Thus, when the wheel speed signal is subjected to a frequency analysis, a peak of the coupled resonance vibration (resonance peak) appears at the lower frequency-side in the case of a decreased pressure than in the case of a normal pressure. This phenomenon appears, due to the above-described characteristic, independent from the tire or vehicle type, the running speed, or the road surface situation for example. Thus, the RFM method focuses on the resonance frequency and issues an alarm when the frequency is determined to be relatively lower than a reference frequency estimated during initialization. Thus, the resonance frequency must be estimated based on wheel speed signals obtained from the ABS provided in a vehicle. The present applicant has previously suggested a method of estimating a resonance frequency (Japanese Patent Application No. 2008-129055 (hereinafter referred to as “the invention of the prior application”)).